The invention relates to synchronous switching power supplies (SPS), particularly for multiple output power supply systems requiring extremely high efficiency and low number of components. SPSs are generally electrical energy regulators which change electrical signals from one form to another by the use of high speed switching components.
The efficiency is improved essentially by reducing the number of semiconductor power devices and inductive parts. The efficiency is further increased by adequately operating the power components. For instance, switching a power transistors at zero voltage and/or current is beneficial. Old fashion SPSs convert AC energy source, e.g. line, into DC voltage which causes inrush current and surge currents every peak of the AC voltage. Pulse width modulation (PWM) is used which results in numerous problems including high circuit complexity, stability problems and significantly higher effective switching frequency. Power factor correction circuits are employed which further increase complexity and decrease efficiency. An exemplary approach to a high efficiency SPS is disclosed in the abovementioned U.S. Pat. No. 4,736,286 entitled "Switching Power Supply" dated Apr. 5, 1988, by the same inventor. This SPS converts AC signal into AC and/or DC signal while employing a minimum number of switching and inductive components. A sinusoidal input current is inherent.
High power SPSs with a plurality of outputs provide one accurately regulated output. The remaining outputs require a further regulation due to a poor tracking of the power transformer and mismatch of power rectifiers. AC output signals are converted into DC signals. Employment of linear regulators results in a very poor efficiency. Switching regulators must be synchronized with the main SPS. A minimum internal load is required to ensure proper operation with no external load.
Conventional SPSs employ an isolating transformer with one output winding for each output of one polarity. A capacitor is coupled to the output via a rectifier. In forward type converters, charging the capacitor results in surge currents which are even higher when a single capacitor is used to supply a plurality of output regulators. The surge currents occur at worst possible time, i.e. when power switches of the main SPS are being turned on.
Low output voltages are commonly desired. Therefore, most of power losses are caused by the forward losses of the output rectifiers. Moreover, snubbers coupled thereacross are required to prevent parasitic oscillations and high voltage spikes during reverse recovery time. One or two diodes are used for each output employing a switching regulator.